Epoxy resins are used in the fields of adhesives, sealants, coating materials, paints, potting materials, molding materials and the like, and most of them are two-pack types. However, since storage or handling of the two-pack type compositions are troublesome, and their pot life is limited, many restrictions are imposed on their application conditions, and they may not be used effectively.
Thus, some one-pack type epoxy resin compositions have been proposed. The known one-pack type compositions are mainly thermosetting ones, and contain BF.sub.3 -amine complexes, dicyandiamide, dibasic acid hydrazide, imidazole compounds and the like, as latent curing agents. Among such latent curing agents, those having good storage stability in sealed containers (in particular at 40.degree. C.) require high temperature (generally from 150 to 215.degree. C.) for curing, while those which can be cured at a relatively low temperature, for example, from 60 to 120.degree. C., have low storage stability. Thus, only few compositions satisfy the storage stability and curing properties at a low temperature at a practical level.
To overcome such drawbacks, following compositions have been proposed:
i) a composition comprising an amine-adduct curing agent,
ii) a composition comprising a curing agent in the form of microcapsules which are dispersed in an epoxy resin,
iii) a composition comprising a curing agent which is adsorbed on molecular sieves for preventing the contact of the curing agent with an epoxy resin.
These one-pack compositions i), ii) and iii) still have technical problems for practically using them, apart from the costs. For example, with the composition i), when the amine-adduct curing agent is solely used, the composition has low adhesion strength, and thus it is not suitable as a one-pack type composition. With the composition ii), the stability of capsules is insufficient. With the composition iii), the composition is unstable against water, and does not have sufficient storage stability.
Accordingly, it has been highly desired to provide a latent curing agent which can cope with both good storage stability, and quick curing properties (in the range between several minutes and 2 hours) at a low temperature (for example, 60 to 120.degree. C.), and which is suitable for a one-pack type epoxy resin composition. That is, the storage stability is important for determining the commercial value of the composition, while the curing properties at a low temperature are valuable properties for saving energy and natural resources. Therefore, it is highly desired to cope with both these properties.